1. Technical Field of the Invention
The present invention relates to automated optical inspection systems, and, more specifically, to a system and method for detecting and classifying semiconductor-wafer surface defects related to the deposition of photoresist during the manufacturing process.
2. Description of Related Art
Optical inspection is a widely used method of non-destructive testing for defects visibly present at or near the surface of an article of manufacture. Optical inspection encompasses a variety of techniques that make use of the patterns produced by energy reflecting off (or passing through) the object being inspected. These reflections constitute an image that can be captured, stored, digitized, examined, compared with other images, and otherwise analyzed. Any defects discovered by the inspection process can then be further analyzed and classified so that, where possible, repairs can be performed either immediately or at some time in the future, and similar defects avoided.
Although applicable in a variety of contexts, the method of the present invention is of particular advantage when applied to the optical inspection of semiconductor wafers during the manufacturing process. Semiconductor wafers are slices of a semiconducting material, such as silicon, that are repeatedly coated, treated, and etched away in selected areas to form very small interconnected electronic devices, such as transistors. A set of thousands, even millions of these interconnected devices is called a xe2x80x9cdiexe2x80x9d. A single wafer can serve as the base for forming several, or even hundreds of such dice. After the dice are populated with electronic devices, they are separated and each is individually encased in a package to form what is commonly referred to as a xe2x80x9cchipxe2x80x9d. Chips can contain a very large number of electrical circuits and are used in constructing a wide variety of electronic devices.
In order to transform a wafer into sets of electronic devices, the wafer undergoes several manufacturing steps. First, a wafer is cut from a crystal ingot (such as crystallized silicon), and an epitaxial layer (a single layer of silicon crystals) may typically be grown on it. The creation of an epitaxial layer is often followed by the growth of high quality oxides on the wafer surface in a process called oxidation. Next, the wafer undergoes several fabrication steps. Each fabrication step places a layer of ions or other materials into or on the wafer, or removes portions from it, in a predetermined geometric pattern so as to form a portion of an electronic circuit.
Common wafer fabrication steps include chemical vapor depositions (CVD), plasma-enhanced vapor depositions (PECVD), etches, ion implantations, diffusions, metalizations, or the growth of structures directly on the wafer. Naturally, these structures are quite small, and the successful completion of the fabrication steps depends largely on the ability to precisely control the geometric placement of gasses, ions, metals, or other deposition materials. The processes of etching, implanting, etc., must be done with sub-micron precision. The precise placement of ions, metals, gasses, or other deposits and removal of other materials is often achieved through a process called photolithography.
Photolithography is a process by which the wafer surface is selectively covered with a material called xe2x80x9cphotoresistxe2x80x9d (or simply xe2x80x9cresistxe2x80x9d) so that subsequent processes of ion implantation, etching, etc., effect only certain areas.
Photoresist is a light-sensitive material that is applied to the entire wafer surface, which is often spun rapidly to distribute the photoresist material evenly across the surface. The photoresist is then selectively exposed to a patterned light source at a predetermined wavelength. The mask""s pattern, like a photographic negative, is projected onto one portion of the wafer at a time by a precision optical device known as a xe2x80x9cstepperxe2x80x9d, and the pattern is preserved on each die by the photoresist. There are different kinds of photoresist used in wafer manufacture, each having different properties. xe2x80x9cPositivexe2x80x9d photoresist, for example, is made soluble by exposure to the light, while xe2x80x9cnegativexe2x80x9d photoresist is hardened.
The next step in photolithography is called development, where the wafer is flushed with a solvent that washes away certain portions of the photoresist. Different types of solvents can be used. One solvent will wash away the portions of positive photoresist that were exposed to the light, while another washes away the unexposed portions of negative photoresist. In either case, the development process leaves the geometric pattern of the mask (or its negative) on each die. The result is a series of xe2x80x9cphotoresist structuresxe2x80x9d that together constitute a developed photoresist layer.
By selectively covering portions of the semiconductor wafer with photoresist structures, the entire wafer can, in a subsequent fabrication step, be exposed to various chemicals, ions, metals, or etchings without affecting the entire areas under the photoresist structures. After each fabrication step has been completed, a wash step is executed. In the wash step, all remaining photoresist is washed away and the wafer is cleaned. Often, one or more additional fabrication steps will be needed, and, the wafer will then undergo further photolithography processes.
As can readily be seen from this discussion, in order to correctly manufacture microelectronic devices, geometrically correct patterns of photoresist structures must be deposited on the wafer during fabrication. And correct geometric patterning is dependent upon properly imaging and developing photoresist layers.
Each fabrication step is expensive and adds significantly to the cost of the semiconductor wafer. Furthermore, fabrication steps such as etching and ion implantation are difficult, if not impossible, to reverse in any cost-effective way. By contrast, photoresist structures can be removed quickly and with minimal disturbance to the underlying wafer structures. Thus, it is desirable to detect defects in the developed photoresist prior to performing a fabrication step. Photoresist defects are those anomalies that will result in impaired or altered electrical characteristics when fabrication of the die is complete, causing it to be rejected. Common photoresist defects include alignment errors, missing photoresist structures, contamination, and skewed photoresist (such as streaking or ring anomolies).
If a defect can be detected in the developed photoresist layer prior to a fabrication step, one simply washes away the photoresist structures and develops another photoresist layer in place of the defective one. If the number of defects attributable to imperfections on the photoresist can be thereby reduced, the corresponding increase in die yield will result in considerable savings.
As mentioned above, the most common method used to detect imperfections in a developed photoresist layer is optical inspection. Other methods often used include electronic, ion beam, and X-ray imaging, although they are slower and more expensive than optical inspection because these imaging techniques illuminate and reconstruct only one point at a time. Laser imaging techniques that capture and compare the angle of reflection of laser beams can also be employed, but sacrifice comparable precision in reporting the position of defects. At any given wafer fabrication facility, however, there are likely to be a multitude of different inspection systems in use, each chosen for a specific purpose after weighing the costs and benefits relative to that particular function.
These numerous inspection systems are only one part of an overall quality control system. The other quality-control components in use may vary from site to site, but often include one or more review systems (through which a more detailed inspection can be performed), analysis and evaluation systems, repair systems, and databases for storing defect-related data.
Although the present invention may be utilized advantageously in numerous applications, the need for the present invention is apparent in the context of a typical optical wafer inspection system. Given the number of images that must be captured for adequate quality control in the manufacturing process, and the detail required for satisfactory inspection, the amount of data and the time it takes to transform and evaluate it can become enormous. The present invention provides an automatic inspection system and method that is both accurate and efficient is the detection and classification of defects.
The present invention provides a system and method for use in inspecting for defects on a structure-bearing surface of an object, for example, in photoresist structures on a semiconductor wafer. In one aspect of the invention, the method includes the steps of receiving data that has already been collected and converted to digital format, assigning a slope value to each pixel, and evaluating an arrangement of the pixel-slope data to locate anomalies. The method may also include the step of creating a visual graphic representation of the processed data.
In another aspect, the present invention is directed to a system for detecting defects on structure-bearing surfaces that performs the method of processing and analyzing the image data.